I don't have enough practice as Protoss, and playing often had situations when opponent retreats... and I can't catch him. That happens:

 - vs zergs: lings just go back
 - terran: marines/marauders kite zealots
 - protoss: ... don't happen often vs protoss, but still some stalkers run away when I could catch them with zealots...

Ok, I've seen how people make a forcefields line and catch however trying to escaps: lings, roaches, zealots, etc... 

I tried to do the same, but it looks like I'm doing something wrong. Usually in early game my composition is stalkers+zealots+guardians. And if I make a FF line my zealots can kill caught units... but the new problem appear: once zealots come to kill those guys, they are in turn become in a range of other opponent units... and are been killed. As a result there are not much value to make a FF line...

Probably I should not get much zealots when I build FF line?
Or may be FF makes sense only if I have MORE army...?

Could you please recommend good guidelines how to play with guardians and/or use FF properly?

Thank you.

P.S. Those materials that I've seen have just general information and don't help me a lot.